This invention relates to regulating an auxiliary output voltage derived from a switching regulator. Specifically, this invention relates to providing a simple, efficient means of accurate regulation for an auxiliary output voltage derived from an over-winding on the primary "buck" inductor of a synchronous buck regulator.
One class of transformer-coupled switching regulators is the coupled-inductor buck converter. This class is identified in Schwartz U.S. Pat. No. 5,552,695 (hereinafter the "'695 patent"), which is hereby incorporated by reference in its entirety. Previously known coupled-inductor buck converters provide multiple regulated outputs by generating a primary output (e.g., 3.3 volts) using a buck converter and adding an auxiliary winding which operates as a coupled inductor for a secondary output (e.g., 5 volts). The basic over-wind topology is well understood in the art, and widely used.
FIG. 1 shows one example of this type of converter 100. In FIG. 1, the over-wind technique produces a crudely regulated auxiliary output voltage 106 capable of providing a voltage that is proportional to the main output voltage 104. The proportionality depends on the number of turns in the primary inductor 160 and secondary inductor 170.
When transistor 110 turns ON, energy builds up and is stored in the primary inductor 160 of the transformer. The phasing of the transformer is chosen so that the voltage produced in secondary inductor 170 at this time reverse biases the diode 130.
When transistor 110 shuts OFF, some of the energy stored in the primary inductor 160 is transferred to the secondary inductor 170. Transistor 120 turns ON substantially simultaneously to transistor 110 shutting OFF. At this point, the diode 130 is forward biased, and a current flows through the secondary inductor 170, thereby charging capacitor 140. A decrease in secondary current occurs with time, until the next cycle begins.
Basic voltage regulation in the circuit is accomplished by transmitting main output feedback 180 to PWM controller 102. After comparing the feedback from main output feedback 180 to a pre-determined voltage level, PWM controller 102 then adjusts the duty cycle of switches 110 and 120 as required.
Schwartz, in the '695 patent, teaches replacing diode 130 with a synchronous MOSFET switch 210, as shown in FIG. 2. Switch 210 (which could be placed alternatively between the inductor and the auxiliary output voltage) is driven synchronously with transistor 120. This modification significantly improves the efficiency of the circuit because of the significantly lower voltage drop of switch 210 when compared with diode 130.
While Schwartz's method offers an improvement in performance of the auxiliary output voltage 106, and the circuits described in the '695 patent works well, regulation accuracy of the auxiliary output voltage 106 has certain obstacles to overcome. A large percentage of the regulation errors in the auxiliary output 106 are caused by voltage drops which occur in the winding resistance (and core losses) of primary inductor 170. As load increases on the main output voltage 104, the voltage drop in primary inductor 160 increases because of the increased current flowing through the winding. Therefore, the auxiliary output voltage 106, which is proportional to the voltage across primary inductor 160, tends to vary directly with load on the main output voltage 104. There are also losses in secondary inductor 170 which increase as a function of load on auxiliary output voltage 106. As a result, auxiliary output voltage 106 varies inversely proportionally to its load.
A popular solution to the problem is to raise the nominal output voltage of the auxiliary output by a volt or two and add a linear regulator to the output. The obvious problems are a reduction in efficiency and the attendant thermal problem of cooling the linear regulator.
In view of the foregoing, it would be desirable to provide a voltage regulator having a primary output and at least one auxiliary output such that each auxiliary output is regulated independently of the regulation of the primary output.
It would also be desirable to provide a voltage regulator having a primary output and at least one auxiliary output such that each auxiliary output is regulated independently of changes to the load on the primary output.
It would also be desirable to provide a voltage regulator having a primary output and at least one auxiliary output such that each auxiliary output is regulated independently of changes to the load on the auxiliary output.